In the production of oil from subterranean reservoirs, it is usually possible to recover only a small fraction of the total oil present in the formation by so-called primary recovery methods which utilize only the natural forces present in the reservoir. To recover oil beyond that which is produced by primary methods, a variety of supplemental production techniques have been employed. In these supplementary techniques, commonly referred to as enhanced recovery operations, a fluid is introduced into the oil-bearing formation in order to displace oil to a production system comprising one or more production wells. The displacing or "drive" fluid may be an aqueous liquid such as brine or fresh water, a gas such as carbon dioxide, steam or dense-phase carbon dioxide, an oil-miscible liquid such as butane, or an oil and water-miscible liquid such as an alcohol.
In miscible flooding operations, it is important to be able to determine the efficiency with which oil can be displaced from porous reservoir rock. Previous attempts at identification and quantification of pore systems and their attributes that effect residual oil and recovery efficiency during miscible flooding have been unsatisfactory, particularly in carbonate reservoir rocks, due to extreme complexity and heterogeneity.